1. Field of the Invention
The present invention relates to a technology for keeping a cumulative dispersion of an optical signal transmitted through an optical transmission system, which includes a plurality of optical add/drop multiplexers (OADMs) with dispersion compensators, within a predetermined range.
2. Description of the Related Art
Recently, an optical transmission system having 10 Gbit/s (10 gigabits) as a bit rate is progressively commercialized in the optical communication field. However, there is still a strong demand from users for the provision of a conventional low-cost 2.4 Gbit/s (2.4 gigabits) optical transmission apparatus.
A technique of an error correction code of an optical signal (a forward error correction (FEC) technique) is not conventionally used in the 2.4 gigabit optical transmission system. Therefore, a transmission distance of an optical signal based on an optical signal-to-noise ratio (SNR) is limited to about 600 kilometers. Regarding optical signals used in the 2.4 gigabit optical transmission system, residual dispersion tolerance that expresses a permissible range of dispersion of the optical signals is as large as about 10,000 ps/nm. At the maximum transmission distance of about 600 kilometers, cumulative dispersion does not exceed the residual dispersion tolerance. Consequently, it is not necessary to compensate for dispersion of the optical signals. In other words, the transmission distance of the 2.4 gigabit optical transmission system is limited by the optical SNR, and is not limited by the chromatic dispersion.
On the other hand, in the 10 gigabit optical transmission system, chromatic dispersion occurs in excess of the residual dispersion tolerance at the maximum transmission distance of optical signals based on the optical SNR. Therefore, dispersion compensation is necessary for the optical signals. A method of uniformly compensating for dispersion is available by adopting dispersion compensators, at each repeater (for example, see Japanese Patent Application Laid-Open No. H11-331074).
FIG. 13 is an explanatory diagram of a configuration of a system that compensates for chromatic dispersion in the conventional 10 gigabit optical transmission system. In the optical transmission system shown in FIG. 13, a regeneration repeater (REG) 1201 is disposed on a transmission path 100. Each time an optical signal is input to the REG 1201, the REG 1201 converts the optical signal into an electric signal, thereby correcting a collapse of a waveform due to noise and dispersion. The REG 1201 converts the electric signal obtained by the conversion from the optical signal into an optical signal again, and transmits this optical signal to the transmission path 100.
Plural optical add/drop multiplexers (OADM) 1202 are provided as nodes on the transmission path 100. Each OADM 1202 drops an optical signal from the transmission path 100, and externally adds an optical signal to the transmission path 100. The OADM 1202 includes a dispersion compensation fiber (DCF) 1203 as a dispersion compensator, and can compensates for dispersion of optical signals.
FIG. 14 depicts a relationship between dispersion compensation and residual dispersion tolerance in the optical transmission system shown in FIG. 13. In FIG. 14, the vertical axis represents cumulative dispersion D [ps/nm], and the horizontal axis represents the number of spans. The number of spans means the number of transmission path fibers between the nodes counted from a starting point. Nodes include repeaters such as linear repeaters and OADMs. In the examples shown in FIG. 13 and FIG. 14, the nodes are the OADMs 1202 on the transmission path 100 from a point R1 to a point R2.
In FIG. 14, a solid line N in a sawtooth shape represents cumulative dispersion of optical signals at each transmission position. A shaded part represents residual dispersion tolerance T that expresses a permissible range of dispersion of optical signals. With a shortest transmission distance, the residual dispersion tolerance T is about 1,000 ps/nm. When optical signals that are input to the regeneration repeater 1201 or each OADM 1202 are not within the range of residual dispersion tolerance T, these optical signals are not recognized or are received as error signals. It is desirable that all optical signals are within the range of the residual dispersion tolerance T in the whole transmission sector. Particularly, all optical signals are desirably within an optimum range of residual dispersion at R2. To achieve this, the DCF 1203 shown in FIG. 13 compensates for dispersion for each span. At each of 1, 2, . . . , m−1, and m that represent the numbers of spans on the horizontal axis, a vertical solid line represents dispersion compensation in each node. These vertical solid lines indicate that cumulative dispersion changes in each node.
However, recently, the FEC technique is also introduced in the 2.4 gigabit system. Since a maximum transmission distance of optical signals limited by the optical signal-to-noise ratio increases based on the FEC technique, dispersion compensation becomes necessary. In other words, the cumulative dispersion of optical signals that are transmitted to the maximum transmission distance exceeds the residual dispersion tolerance. Consequently, dispersion compensation of the optical signals becomes necessary.
FIG. 15 is an explanatory diagram of the conventional 2.4 gigabit optical transmission system. FIG. 16 is an explanatory diagram of a method of compensating for chromatic dispersion in the conventional 2.4 gigabit optical transmission system. In the 2.4 gigabit optical transmission system shown in FIG. 15, dispersion compensation is not carried out. In the 2.4 gigabit optical transmission system shown in FIG. 16, dispersion compensation is carried out using the DCF 1203. In FIG. 15 and FIG. 16, like reference numerals designate like constituent elements as those shown in FIG. 13, and therefore, redundant explanation is omitted.
FIG. 17 depicts a relationship between dispersion compensation and residual dispersion tolerance in the optical transmission system shown in FIG. 15 and FIG. 16. In FIG. 17, the vertical axis represents cumulative dispersion D [ps/nm], and the horizontal axis represents the number of spans. A solid line N1 at an upper part of the diagram expresses dispersion of optical signals in FIG. 15, and a solid line N2 in a sawtooth shape at a lower part expresses dispersion of optical signals shown in FIG. 16. Residual dispersion tolerance T shown by hatched lines becomes about 16,000 ps/nm, and this width is 16 times that of the 10 gigabit optical transmission system.
Regarding the cumulative dispersion N1 shown in FIG. 17 where dispersion compensation based on the configuration shown in FIG. 14 is not carried out, the cumulative dispersion exceeds the residual dispersion tolerance T when the distance increases from R2 to R2′. In this case, the DCF 1203 is disposed in all the OADMs 1202 in all nodes as shown in FIG. 16, thereby reducing the cumulative dispersion N2. However, compensation carried out by each DCF 1203 is set to a small level. Cost of the optical transmission system depends on the number of disposed DCFs 1203. Therefore, the dispersion compensation method of disposing the DCFs 12 in all the OADMs 1202 in all nodes as shown in FIG. 16 increases cost.
According to the technique disclosed in Japanese Patent Application Laid-Open No. H11-331074, dispersion compensation is carried out in a point-to-point transmission, by having one transmission end and one reception end fixed for optical signals. Recently, multiple functions of nodes (relay points) are required in the optical transmission system. Optical signals are added or dropped at any position on the transmission path such as an OADM, a wavelength cross connect (an optical hub). Therefore, even when an optical signal is added from any node or dropped from any node, cumulative dispersion needs to be accommodated within residual dispersion tolerance.